Liquid crystal displays are commonly used as display devices for compact electronic apparatuses, not only because they provide good quality images but also because they are very thin. Liquid crystal in a liquid crystal display does not emit any light itself. The liquid crystal requires a light source so as to be able to clearly and sharply display text and images. Therefore, a typical liquid crystal display generally requires an accompanying backlight module. If a cold cathode fluorescent lamp (CCFL) is used in a backlight module, the backlight module generally includes a backlight control circuit. The backlight control circuit is configured for converting a direct current voltage to an alternating current voltage to drive the CCFL.
Referring to FIG. 3, a typical backlight control circuit 100 includes a pulse width modulation (PWM) circuit 110, a frequency setting circuit 140, an inverter 120, and a lamp 130. The PWM circuit 110 is configured to generate a pulse control signal, and output the pulse control signal to the inverter 120. The inverter 120 is configured to convert an external direct current voltage to an alternating current voltage to drive the lamp 130 under the control of the pulse control signal. The frequency setting circuit 140 is configured to set a frequency of the pulse control signal outputted by the PWM circuit 110.
The PWM circuit 110 includes a working frequency capacitor terminal 111, a working frequency resistor terminal 112, and a startup frequency resistor terminal 113 for setting a frequency to light the lamp 130.
The frequency setting circuit 140 includes a capacitor 141, a first resistor 142, and a second resistor 143. The capacitor 141 is connected between the working frequency capacitor terminal 111 of the PWM circuit 110 and ground. The first resistor 142 is connected between the working frequency resistor terminal 112 and ground. The second resistor 143 is connected between the working frequency resistor terminal 112 and the startup frequency resistor terminal 113. A capacitance of the capacitor 141 can be 220 picofarads (pF). A resistance of the first resistor 142 can be 52.3 kiloohms (KΩ). A resistance of the second resistor 143 can be 240 kiloohms.
The PWM circuit 110 can be an OZ960 type IC. The frequency of the pulse control signal outputted by the PWM circuit 110 for lighting the lamp is determined by the capacitor 141 and the first and the second resistors 142, 143 of the frequency setting circuit 140. The frequency of the pulse control signal can be calculated according to the following formula (1):
                              f          s                =                                            70              ×                              10                4                                                    C              ×              R                                .                                    (        1        )            In formula (1), “fs” denotes the frequency of the pulse control signal, and a unit of the pulse control signal is kilohertz (KHz). “R” denotes the resistance of the first resistor 142 and the second resistor 143 connected in parallel with the first resistor 142, and a unit of the resistance is kiloohms. “C” denotes a capacitance of the capacitor 141, and a unit of the capacitance is picofarads.
When the backlight control circuit starts to work, a startup frequency for lighting the lamp 130 is a frequency of the alternating current voltage outputted by the inverter 120, and is the same as the frequency of the pulse control signal. In general, because the capacitance of the capacitor 141 and the resistances of the first and second resistors 142, 143 are fixed, the frequency of the alternating current voltage outputted by the inverter 120 and the frequency of the pulse control signal are fixed. Thus, the startup frequency for lighting the lamp 130 is fixed.
However, under different environment temperatures, the lamp 130 has different equivalent resistances which correspond to different optimal startup frequencies. In general, the startup frequency of the lamp 130 increases with a decrease in the environment temperature. The lamp 130 can be lighted up when the lamp 130 is driven with a frequency approximately the same as the optimal startup frequency. When the environment temperature changes to a low temperature, the actual startup frequency of the lamp 130 remains the same and thereby is lower than the optimal startup frequency. Thus it can be difficult light up the lamp 130.
Therefore, a new backlight control circuit that can overcome the above-described problems is desired. What is also desired is a method for controlling lighting of a lamp using such backlight control circuit.